This invention relates to a tibial component for a replacement knee prosthesis and can be used with the natural condyle bearing surfaces of a co-operating femur or with artificial condyles provided by a femoral prosthesis.
Tibial components comprising a metal tibial tray with removable and interchangeable synthetic plastics material bearing surface components of different thicknesses are available, the alternative bearing surfaces being provided so that the correct ligament tension can be achieved.
The femoral component of a surface replacement knee prosthesis is positioned and orientated mainly relative to the bone itself. There are three inner position variables: forward/backward, side to side, and vertically and there are three orientation variables: flexion/extension, varus/valgus, and rotation about the femoral shaft centerline. Additionally, the profile of the tibial condyle depressions affects the stability and locus of knee motions.
The tibial component is similarly positioned and orientated relative to the tibia.
When these components have been installed the only way a surgeon can at present adjust the knee is by choosing a bearing surface of different thickness to create the desired ligament tension or by removing the components and retrimming the bones.
If a surgeon therefore finds he has made a mistake, or changes his mind about the optimum knee alignment, there is no simple adjustment to be made. Furthermore, existing knee designs force a compromise in the horizontal positioning of both femoral and tibial components. The fixation must lie on or close to the outer cortical walls of the bone but this may not suit the ideal position for the bearing surface.